Each baby is different, but most 9 month olds still need 3 naps a day: a good morning and mid-day nap, and a late afternoon cat nap usually. I have worked in child care for many years, mostly with infants and toddlers, and most of the toddlers need at least 2 naps during the day until around 14 months.
Babies who are sleep deprived aren't able to learn as much when they are so tired. A baby who is sleep deprived can also seem over-active, as the adrenaline and hormones in their body start to kick in and keep them awake when they are desperately needing sleep.
Please make sure your baby is getting adequate sleep. Don't cut his naps back until he signals that he is ready (like when he starts to not go down for a nap on his own at his usual time). And when he does show he's ready, make is a slow transition, like more naps some days, and less naps other days, depending on what he needs that day.
A well-rested baby sleeps better than a sleep deprived baby, because of the hormones that I described above. Sleep is sooo important for his development. HIs brain is working wonders while he sleeps so let him sleep as much as he needs.